There's no standard SPARQL function that will take a string that represents a number in hex and convert it to a string in decimal notation (or, for that matter, to an xsd:integer). I don't believe you could compose such a function using standard SPARQL built-in functions, either, as you'd need to loop over the characters in the hex representation to do the conversion and there's no looping/mapping standard function in SPARQL.

So, you could write your own custom function if your implementation supports it (as per your other question).